The present invention is directed to a method for forming ferroelectric integrated circuits and, in particular, ferroelectric capacitors.
Ferroelectric capacitors are typically formed through a series of deposition and defining steps. The process requires that several oxygen anneals be done. Oxygen anneals are required in order to form correct crystal phases of deposited sputtered or spun-on "PZT" and to reduce electrode and material deficiencies.
"PZT" is the name for a ferroelectric material comprising lead zirconate titanate and having the general formula Pb(Ti.sub.x Zr.sub.1-x))O.sub.3 wherein x=0 to 1. As deposited, sputtered, or spun-on PZT is amorphous and has no (or insufficient) ferroelectric properties. Annealing in oxygen is necessary to form the correct crystallographic phases that produce the desired ferroelectric properties. For example, when PZT is used in memory circuits, the desired ferroelectric phase is a tetragonal phase. One of the desired ferroelectric characteristics is then a permanent dipole moment without an applied electric field. This can only occur if there is a unilateral displacement of the positively charged Ti.sup.+4 ion against its negatively charged O.sup.-2 surroundings. Oxygen vacancies are quite likely to occur in the sputtered PZT material due to target imperfections and due to oxygen reactivity. Thus, oxygen is needed to repair these defects and ensure good ferroelectric behavior. These oxygen anneals also condition the electrode/PZT interface by acting as an electrical acceptor atom that helps reduce the excess charges at the interface that are generated as a result of material lattice mismatch.
Currently, oxygen anneals are done in an O.sub.2 ambient at a temperature greater than 500.degree. C. Typically, the oxygen anneal is done using a furnace anneal or rapid thermal annealing process (RTA).
Unfortunately, the effect of these anneals can be reduced, or even eliminated, by some of the other processing steps used to form the ferroelectric capacitor. For example, many of the subsequent integrated circuit processing steps involve low pressure, weakly ionized and highly energetic gas conditions (known as plasmas). Medium energy (&lt;1 keV) electrons and photons are produced in these plasmas. These particles can ionize in the ferroelectric material to form electron/hole pairs or can ionize constituent PZT atoms. This extra charge produced as a result of these processes can accumulate to form internal electric fields larger than and/or in opposite directions to that of the induced structure dipole moment in the ferroelectric material.
For example, a SiH.sub.4 glass deposition step can result in the H.sub.2 or N.sub.2 becoming substitutional impurities in the ferroelectric crystal which can destroy the crystal's ferroelectric effect. If sufficient H.sub.2 is accumulated substitutionally in the ferroelectric crystal, the induced structural dipole moment reduces to zero, and the ferroelectric hysteresis curve collapses to that of a conventional linear dielectric medium. This could be interpreted as a region of greatly reduced resistivity embedded in the ferroelectric material.
The object of the present invention is to provide an improved method for forming a ferroelectric capacitor which does not suffer from the drawbacks described above.